Channel routers are how Notifier determines which channel and transport (i.e, the destination/service) to send to, by notification and recipient context.

Channel routers are an ordered (by priorities) set of services, each notification and recipient combination runs through all channel routers. Channel routers act as a sieve, where all possible channels for the recipient and notification are initialized, then each channel router has an opportunity to modify those channels.

Without any configured channel routers, a message would be sent ot all channels. However, notifier provides a few built in channel routers.

There is no configurability for channel routers, service parameter or otherwise, except by importance. Instead, if further control is desired, alter channel routers by removing channel router services or changing weights in a compiler pass.

Built-in channel routers

Notification -object channel

When sending a notification, you have an opportunity to specify which channel it should be sent to. Usually this shouldn't be used as sender code shouldn't concern itself with routing, unless the sender code is also owned by the site, ie. not contrib.

$notification = (new Notification())
  ->subject('Subject!')
  ->content('Content!')
  ->channels(['sms']); 

Channel policies

Class: \Drupal\notifier\ChannelRouting\Policy, priority: -2048

Channel policies narrow channels for a notification and recipient combination by the importance -level of a notification.

Notifier module uses policies as a sieve. All channels are made available, then policies are applied depending on a notifiers importance.

INFO

This behavior is different to channel policies in Symfony Frameworks' Notifier component, where Framework uses policies as a starting point for where notifications are sent.

Notifier module also allows you to disable policy behavior entirely, or per importance, by setting an importance level to ~ (NULL) in service parameter configuration.

$notification = (new Notification())
  ->subject('Subject!')
  ->content('Content!')
  ->importance(\Symfony\Component\Notifier\Notification\Notification::IMPORTANCE_URGENT); 

notifier.channel_policy:
  urgent: ['chat']
  high: ~
  medium: ~
  low: ~

In this example, the notification would be routed to chat channel, since it has the urgent importance. If the notification had a different importance level, the policy would not make any changes.

Supports method

Class: \Drupal\notifier\ChannelRouting\Supports, priority: -32768

An internal channel router ensures a channel supports the notification and recipient combination by invoking the supports() method on a channel.

WARNING

The Supports method channel router runs late, and is a form of safety. Removing or reorder with caution.

Custom channel routers

Creating a custom channel router to determine which channel a notification and recipient combination should be routed to is a common occurrence.

services:
  _defaults:
    autoconfigure: true
    autowire: true

  Drupal\my_module\ChannelRouting\MyChannelRouter: ~

namespace Drupal\my_module\ChannelRouting;

use Drupal\notifier\ChannelRouting\Attribute\AsChannelRouter;
use Drupal\notifier\ChannelRouting\ChannelRoutingInterface;

#[AsChannelRouter(priority: 256)]
final class MyChannelRouter implements ChannelRoutingInterface {
  public function channels(Channels $channels, Notification $notification, RecipientInterface $recipient): void {
    // Modify $channels object, with $notification and $recipient as the context.
  }
}

Some recipients provide entity context, you can check this by type asserting with \Drupal\notifier\Recipients\Recipient\EntityRecipientInterface.

public function channels(Channels $channels, Notification $notification, RecipientInterface $recipient): void {
  if ($recipient instanceof EntityRecipientInterface) {
    $entity = $recipient->getEntity();
  }
}